With the advent of modern papermaking techniques, paper machine speeds and width have increased to more than 5,000 feet per minute and 200 inches respectively. As speeds and widths increase, the difficulty in making a product with uniform machine- and cross-machine (c.d.) direction profiles increases concurrently. In the manufacture of quality tissue or towel products for wiping purposes, control of the c.d. basis weight profile is important to produce consistently uniform products acceptable to the customer in both feel, performance and sight characteristics.
Nonuniformities in the c.d. profile are primarily caused by irregularities in the slice opening, the opening in the headbox from which the papermaking furnish exits. It is known in the art to provide means to deform one or both of the lips forming the slice opening in order to adjust the basis weight profile to even out undesirable c.d. variations. These lip-deforming means usually have taken the form of mechanical devices, such as rods or spindles, secured to one of the lips. By affixing these rods at selected intervals across the width of the lip, one or more selected rods may be actuated up or down in order to open or close that portion of the slice opening as desired.
The devices described above function adequately for so-called Fourdrinier type machines, or single wire machines in which the paper stock is laid on a single moving wire beneath the headbox. However, the newer generation high speed machines often utilize a "twin wire" or "crescent former" configuration in which the papermaking stock is introduced in a jet into a forming zone in a hydraulic nip formed between two converging wires or a converging wire and felt. In certain designs, when the inlet lips have to be positioned as close as possible into the forming zone, space is at a premium and such mechanical devices are too cumbersome and difficult to adjust to be used effectively. One such inlet is the Kimberly-Clark Turbulent Slot or Parallel Lip design, explained below.
The construction of the headbox channel immediately upstream from the slice opening has been effected in different ways to produce different results. Certain papermakers and manufacturers build headboxes with upper and lower stock flow channels in the lips converging toward the slice opening, thereby producing a uniform stock jet issuing from the headbox. Other papermakers have chosen to construct headboxes with upper and lower stock flow channel walls parallel to one another, thereby producing a turbulent stock jet which proportedly provides better mixing of the fibers in the stock jet. A benefit incident to the converging wall type inlet is that the stock jet issuing from the headbox remains intact for a considerable distance after exiting the headbox. Therefore, in a twin wire machine or a crescent former machine, the headbox can be backed away from the nip between the wires and the stock jet may be "shot" into the nip. By contrast, the stock jet issuing from a parallel-lip headbox disintegrates soon after leaving the headbox, so that the slice opening must be placed within the forming zone very near the nip.
In U.S. Pat. No. 2,928,464, Western et al, it is proposed to provide a headbox of the converging wall type with flexible slice lips backed by rigid support members, with expandable bags or capsules placed therebetween which deflect the flexible lips at appropriate points. The bags are fluid filled and connected to a bellows or pump so that the pressure may be varied to open or close the slice opening at any particular point along its width. The flexible slice lips require very little pressure exerted thereon to effect the deflection desired due to the flexible nature of the slice lips. Therefore, the bags are constructed of rubber, polyethylene, polyvinyl chloride or other suitable flexible materials, which is sufficient to withstand the low pressure levels required.
However, in the headbox utilized by Applicant, in which the slice lips are rigid members extending a substantial distance beyond the headbox support members into the forming zone, and which the support members and the slice lips are relatively thin, such devices would be unacceptable.
Therefore, there is an immediate need for means which are suitably compact to satisfy the limited space requirements of a twin wire or crescent forming system, yet having the capability to deflect a relatively rigid slice lip in order to correct c.d. basis weight nonuniformities.